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  <section id="unit-systems">
<h1>Unit systems<a class="headerlink" href="#unit-systems" title="Permalink to this headline">¶</a></h1>
<p>This module integrates unit systems into SymPy, allowing a user choose which
system to use when doing their computations and providing utilities to display
and convert units.</p>
<p>Unit systems are composed of units and constants, which are themselves
described from dimensions and numbers, and possibly a prefix. Quantities are
defined by their unit and their numerical value, with respect to the current
system.</p>
<p>The main advantage of this implementation over the old unit module is that it
divides the units in unit systems, so that the user can decide which units
to use, instead of having all in the name space (for example astrophysicists
can only use units with ua, Earth or Sun masses, the theoricists will use
natural system, etc.). Moreover it allows a better control over the dimensions
and conversions.</p>
<p>Ideas about future developments can be found on the <a class="reference external" href="https://github.com/sympy/sympy/wiki/Unit-systems">Github wiki</a>.</p>
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<li class="toctree-l2"><a class="reference internal" href="philosophy.html#dimensions">Dimensions</a></li>
<li class="toctree-l2"><a class="reference internal" href="philosophy.html#quantities">Quantities</a></li>
<li class="toctree-l2"><a class="reference internal" href="philosophy.html#the-need-for-a-reference">The need for a reference</a></li>
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<li class="toctree-l1"><a class="reference internal" href="dimensions.html">Dimensions and dimension systems</a></li>
<li class="toctree-l1"><a class="reference internal" href="prefixes.html">Unit prefixes</a></li>
<li class="toctree-l1"><a class="reference internal" href="unitsystem.html">Units and unit systems</a></li>
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<li class="toctree-l2"><a class="reference internal" href="quantities.html#module-sympy.physics.units.util">Conversion between quantities</a></li>
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